(1) Field of the Invention
The invention relates to an arrangement for correcting the pulse distortion caused by echo signals and intersymbol interference, particularly for use in transmission systems for simultaneous two-way transmission of homochronous data signals over two-wire transmission paths.
(2) Description of the Prior Art
Transmission systems for information signals are usually formed by a combination of two-and four-wire connections. Four-wire connections are formed by two one-way paths, namely by a one-way send path and a one-way receive path. A two-wire connection is formed by a two-way path over which signals can be transmitted simultaneously in mutually opposite directions in one and the same frequency band. These different paths are interconnected by means of a hybrid.
As is known, a hybrid is formed by a four-port circuit. A first port, what is commonly referred to as the send port is connected to the one-way send path. A second port, what is commonly referred to as the receive port, is connected to the one-way receive path; a third port, what is commonly referred to as the cable port, is connected to the two-way path, while a balancing network is connected to the fourth part, what is commonly referred to as balancing port. Said balancing network has for its object to match the hybrid to the cable impedance. When this balancing network is adjusted perfectly, a signal in the send path will appear at the cable port, but will not appear in the receive path. If in contrast therewith a signal is applied over the two-way path to the cable port, then with a perfect adjustment of the balancing network this signal will appear in the receive path but not in the send path.
Owing to the diverse cable lengths and cable types which may form the two-way path connected to the cable port, the cable impedance is usually not known accurately, so that the balancing network can never be adjusted perfectly. The result is that a portion of the signal occurring in the send path reaches the receive path via the hybrid.
As every two-way path usually comprises one or more discontinuities against which a signal may partly reflect, a reflected version of the signal occurring in the send path may, furthermore, be expected to appear in the receive path. The two parts of the signal occurring in the send path, which occur in the above-described manner in the receive path will, together, be designated, as is customary, the echo signal and will result in the occurrence of pulse distortion in the received signal.
The quality of the received signal is not only negatively influenced by the occurrence of the echo signal reaching the one-way receive path in the above-described manner, but also by the presence of a second cause of pulse distortion in this received signal. Said pulse distortion is the result of the non-ideal transmission characteristic, for pulse transmission, of the two-way path through which the received signal is applied to the cable port of the hybrid and manifests itself as a mutual influencing of the individual data symbols in the received signal. As is customary, this type of pulse distortion will be designated intersymbol interference.
In order to be able to guarantee high-grade information transmission when transmission systems formed by a combination of two- and four-wire connections are used, it is therefore necessary to employ circuits by means of which both the intersymbol interference and the echo signal can be eliminated, or at least the negative influence of both of them on the quality of the signal in the receive path can be significantly reduced.
It is customary that in transmission systems the intersymbol interference is reduced to the best possible extent by arranging in series with the path to be traversed by the received signal an equalizer arrangement the transfer characteristic of which is adjusted in such a manner, automatically or non-automatically, that it becomes equal to the best possible extent to the inverse of the transfer characteristic of the two-way path. In addition, it is customary that the echo signal is reduced as much as possible by the use of circuits which are commonly referred to as echo cancellers.
Reference 1 of paragraph D describes an embodiment of an echo canceller of such a type. The echo canceller described therein comprises a cancelling circuit connected to the one-way send path, by means of which a synthetic echo signal is generated whose shape corresponds to the best possible extent with the echo signal to be expected and which, after digital-to-analogue conversion, is subtracted from the signal present in the receive path, to form a residual signal from which the echo signal has been removed to the best possible extent. This echo cancelling circuit is formed by a random access memory (RAM), consisting of 2.sup.N memory locations, which can be individually addressed by means of an address code consisting of N bits. This address code is obtained from a shift register connected to the one-way send path and comprises the N data symbols which were last transmitted and addresses in the RAM a compensation value in digital form, which after digital-to-analogue conversion is subtracted from the signal present in the receive path in order to cancel the echo signal contained therein. The residual signal thus obtained is used to re-adjust iteratively the contents of the RAM in such a way that the best possible cancellation of the echo signal is achieved.
Although the prior art echo cancellers can eliminate the echo signal in the one-way receive path in a satisfactory manner, or are at least capable of reducing the negative influence thereof to a considerable extent, the resulting residual signal is nevertheless still of an insufficient quality for high-grade data transmission, which is due to the presence of intersymbol interference in said residual signal. In order to obtain a residual signal of a sufficient quality a separate equalizing circuit in series with the path to be traversed by the received signal is still required.